Well just to screw you up more and let you know sizes vary a lot the guy above me is 5'9 and uses a 58; I'm 5'8.5 with long legs and prefer a 52.
My motobecane is an MTB. For what it's worth I was in the middle of two sizes and went up (18) but probably should have went down. The standover was a tad higher than their chart suggested.